1. Field of the Invention
This invention relates to a circuit arrangement for igniting and operating at least two discharge lamps, provided with
input terminals for connection to a supply voltage source, PA1 means I coupled to the input terminals for generating a high-frequency voltage from a supply voltage delivered by the supply voltage source, PA1 a load branch B coupled to the means I and comprising PA1 means II for limiting the voltage across branch A and branch C to a first value during the ignition of the discharge lamps.
a first branch A comprising first terminals for accommodating a discharge lamp and a first inductive element L1, PA2 a second branch C shunting the first branch A and comprising further terminals for accommodating a discharge lamp and a second inductive element L2 which is magnetically coupled to the first inductive element L1, and
2. Description of the Related Art
Such a circuit arrangement is known from U.S. Pat. No. 4,441,054. The known circuit arrangement is suitable for operating two discharge lamps. The first inductive element L1 and the second inductive element L2 together form a balancer transformer. This balancer transformer achieves during lamp operation that the currents through the two discharge lamps are approximately equal. This is important especially when the circuit arrangement offers the possibility of dimming the discharge lamps, since otherwise the luminous fluxes of the discharge lamps may differ considerably in the dimmed state, which is regarded as undesirable in many applications. It is a disadvantage of the known circuit arrangement, however, that with one of the discharge lamps ignited and the other discharge lamp not yet ignited during the ignition phase, a voltage having a very high amplitude is present across said other discharge lamp. Such a very high voltage conflicts with the safety requirements such as, for example, those formulated in IEC 928. A second disadvantage is that in this situation a current with a comparatively high amplitude flows through the inductive element forming part of the branch in which the already ignited discharge lamp is present. The balancer transformer should be dimensioned such that no saturation of the balancer transformer occurs at a result of this current of comparatively high amplitude because otherwise current pulses will arise which will considerably shorten the lives of at least part of the components from which the circuit arrangement is built up. The result of this is that the balancer transformer in the known circuit arrangement is a comparatively voluminous and expensive component.